SUMMARY Methyl mercaptan (methanethiol) is a colorless toxic gas that smells like rotten cabbage. It is considered a high priority chemical threat agent by the CounterACT Program, with its toxicity similar in degree to that of hydrogen sulfide, but less than that of hydrogen cyanide. Like hydrogen sulfide and hydrogen cyanide, methyl mercaptan inhibits cytochrome c oxidase, the terminal enzyme of the mitochondrial electron transport chain, and it thus inhibits cellular respiration. Cyanide and sulfide inhibition of cytochrome c oxidase increases cellular production of reactive oxygen species and reduces ATP generation, but whether methyl mercaptan also induces oxidative stress and reduces ATP generation is not known. Methyl mercaptan is released from decaying organic matter, and can be present in coal tar, crude oil, and natural gas. It is used in several industries, for example in manufacturing plastics and pesticides, and as an additive to jet fuel. Seven workers have been killed in the past several years when they were exposed to methyl mercaptan gas in industrial accidents. Unfortunately, no antidote exists for methyl mercaptan poisoning, and treatment consists of general supportive care. A specific antidote is clearly needed, preferably one that can be administered quickly in the field, for example by intramuscular injection or inhalational delivery. Cobinamide, a vitamin B12 analog, binds hydrogen cyanide and hydrogen sulfide?and thereby neutralizes these toxic gases?and rescues animals from cyanide and sulfide poisoning. We have found that cobinamide also binds and neutralizes methyl mercaptan, and rescues mice from methyl mercaptan poisoning. Cobinamide?s rescue of mice is potentiated by sodium thiosulfate, a well- known cyanide antidote that we found reacts directly with methyl mercaptan. We now propose to determine: (i) the mechanism of methyl mercaptan-induced toxicity, specifically studying if methyl mercaptan induces oxidative stress and reduces ATP production and whether cobinamide and thiosulfate reverse these potential effects of methyl mercaptan; (ii) the optimal cobinamide and thiosulfate doses and the optimal ratio of these two agents that rescue mice from a lethal exposure to methyl mercaptan gas; (iii) the cobinamide and thiosulfate doses that rescue rabbits and pigs from methyl mercaptan poisoning, basing the initial doses of the two drugs and their relative ratio on the mouse studies; and (iv) whether inhaled cobinamide and thiosulfate provide better pulmonary protection and improved survival than intramuscular injection of the drugs. These studies would be required by the Food and Drug Administration in anticipation of developing cobinamide and thiosulfate as a treatment for methyl mercaptan poisoning. Having one antidote that could treat hydrogen cyanide, hydrogen sulfide, and methyl mercaptan poisoning would be highly advantageous.